The present invention relates to solvent extraction, and more particularly to a mixing and separating apparatus and process for recovering metals and other materials from solution.
Solvent extraction involves the selective separation of one or more chemical components of a liquid solution. Although there are many variations in techniques of solvent extraction, the principal method utilized involves mass transfer between two immiscible liquid phases, usually an organic phase and an aqueous phase. Solvent extraction is accomplished by using two liquids in which the component desired to be extracted, termed the key component, has a different solubility and/or is selectively soluble with respect to other dissolved components. A solvent is chosen to contact the key component containing solution, into which the key component is either preferentially or selectively soluble. To enhance both the rate of solution and degree of solubility of the key component in the solvent, chemical complexes may be formed in one or both of the liquid phases, or conventional chemical reactions may occur. Such combinations of mass transfer and complex formation or reaction influence the overall rate of extraction.
The solvent extraction process involves bringing a solvent and a solution containing a key component into intimate contact, separating the two phases, and recovering the key component from the solvent phase. Several factors are involved which affect the overall rate of extraction. Among them are the interfacial area between the two liquid phases, their interfacial surface tension, concentration differentials between the phases, temperature, pressure, and amount of agitation. For example, reducing the drop size of the discontinuous liquid phase dispersed within the continuous liquid phase increases the interfacial area of contact for a given volume. However, the maximization of one factor may have an adverse effect on a different part of the overall process. A highly dispersed system in the mixer stage of the process will increase mass transfer, but at the expense of an increasingly lengthy and expensive separation of the phases later in the process.
Basic equipment for carrying out solvent extraction is well known. The simplest extraction system need have only a mixer followed by a settling tank. However, few extractions can be carried out efficiently with only a single contacting stage. Most solvent extraction operations utilize multiple contact systems in either batch or continuous operation. Contacting has typically been accomplished through use of baffleplate mixers, agitated stirrers, packed towers or plate columns. Separation is done usually by gravity settling or centrifugation.
Recently, motionless mixers have been utilized in solvent extraction operations including recovery of metals from aqueous process streams. These devices are termed motionless mixers because they have no moving parts. Mixing is accomplished by pumping one or more fluids through a conduit containing baffles, spirals, or twisted vanes. For example, Australian Pat. No. 449,240 discloses the extraction and recovery of uranium, copper, cobalt and vanadium from aqueous solutions using a motionless mixing device. Likewise, Henrickson, U.S. Pat. No. 3,942,765, teaches use of an extraction device combining a motionless mixing device having triangular shaped vanes with a settling device to extract and recover copper from a dliute aqueous solution.
However, neither of those references address the problem of maximizing extraction efficiency while maintaining an easily separable dispersion of the solvent and solute phases of a two-phase immiscible liquid system. A need exists in the art for a solvent extraction system in which contacting of the two liquid phases is maximized for maximum extraction efficiency while at the same time, ease of separability of the mixed phases is attained.